raindrops 0.19.0 - Rack 2.x middleware compatibility

This release fixes Rack 2.x compatibility for the few users of
Raindrops::Middleware
<https://bogomips.org/raindrops/Raindrops/Middleware.html>.
Thanks to Dmytro Shteflyuk for this release.

No need to upgrade unless you use Raindrops::Middleware with
Rack 2.x.

There's also a few minor, inconsequential cleanups.

Dmytro Shteflyuk (1):
      Properly override respond_to? in Raindrops::Middleware::Proxy

Eric Wong (2):
      Ruby thread compatibility updates
      tcp_info: remove unnecessary extconf.h include